The agr locus encodes the central regulatory system for staphylococcal pathogenesis and other stress-related functions. It is a quorum-sensing system that contains a two-component signal transduction module, encoded by agrA and C, and a peptide autoinducer (AIP), encoded by agrD, that is the activating ligand. Natural variants exist that cross inhibit agr autoinduction in heterologous combinations, thus blocking pathogenesis. This is the continuation of a long-term program whose overall goal is to understand the mechanism of agr autoinduction, the role of the agr autoinduction circuit in the pathogenesis of staphylococcal disease, the therapeutic potential of agr inhibition, and the biological significance of agr variants and their biotypes. Specific Aims for this period are: 1. To determine mechanisms of peptide secretion, binding, activation and inhibition. 2. To determine the mechanism of signal transduction in the agr system. 3. To determine the therapeutic potential of agr inhibition. Design and Methods. Genetic and biochemical methods will be used to determine the mechanism of AIP secretion. A direct ligand-binding assay will be used to analyze receptor-ligand interactions. Specific ligand binding sites will be identified by mutagenesis and cross-linking studies; the mechanism of receptor activation will be determined by mutational and structural studies. Constitutively active receptor mutants will be an important tool in vitro mechanistic studies. Mutants resistant to inverse agonism will be isolated and analyzed to test the hypothesis that there are specific inhibitory contacts as well as activating contacts. The hypothesis will be tested that administration of an inhibitory AIP at a distal site can block the establishment of an experimental infection, or if administered after an infection has been established, can attenuate or eradicate the infection.